An investigation found that, over three decades, officers trawled birth and death records for suitable matches. Aliases were created based on the dead children's details and officers were issued with driving licences and national insurance numbers.

Some spent 10 years pretending to be people who had died.

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The Met said the practice was not ''currently'' authorised, but announced an investigation into ''past arrangements for undercover identities used by [Special Demonstration Squad] officers''.

The chairman of the British Parliament's Home Affairs select committee, Keith Vaz, said he was shocked at the ''gruesome'' practice. ''It will only cause enormous distress to families who will discover what has happened concerning the identities of their dead children,'' he said.

Two undercover officers provided a detailed account of how they and others used the identities of dead children. One, who went undercover in anti-racist groups, said he felt he was ''stomping on the grave'' of the four-year-old boy whose identity he used.

''A part of me was thinking about how I would feel if someone was taking the names and details of my dead son for something like this,'' he said.

The other officer, who adopted the identity of a child who died in a car crash, said he was conscious the parents would ''still be grief-stricken''. He argued his actions could be justified because they were for the ''greater good''. Both worked for the secretive SDS, which was disbanded in 2008.

Dozens of SDS officers, who posed as anti-capitalists, animal-rights activists and violent far-right campaigners, used the identities of dead children. One document indicated about 80 officers used such identities between 1968 and 1994.

''We are not prepared to confirm nor deny the deployment of individuals on specific operations,'' the Met said.